The present invention relates to the field of electro-magnetic mechanical actuators and more particularly it relates to an actuator that provides a powered toggle stroke between two unpowered but positively stabilized stroke-end conditions, suitable for locking movable components in place by remote control as frequently required in defense ordnance including missiles and other aerospace craft as well as in ground vehicles, marine vessels and in many kinds of buildings such as residential, industrial, and commercial.
Usage of electro-magnetic actuators has continuously expanded as part of the overall technological advancement of communications, electronics, aerospace and defense ordnance of all kinds including missiles. Such actuators play a key role in a wide variety of present day equipment, especially remotely controllable mechanisms in vehicles, spacecraft, aircraft, missiles, boats, ground equipment, public, commercial and residential buildings, garages or parking areas, etc.
Many of these needs are for a basic actuator in coaxial plunger form wherein the only moving portion is an armature fitted with a central pin that the actuator can extend/retract electrically over a designated linear displacement or xe2x80x9cstrokexe2x80x9d. The pin itself can serve directly as a lock bolt, e.g. for a mechanism, door, window, cover, etc., or the pin can be adapted as a drive rod to drive other mechanisms.
Typically, operation can be from a DC (direct current) power source, e.g. a 12 volt battery. Since locks or locking devices are often left unattended for very long period of time, both stroke-end conditions of the actuator""s moving element, typically an armature, should be passive i.e. not consuming any standby electrical power. Furthermore, in these stroke-end locations, the armature needs to be positively stabilized with an adequate amount of holding force; for operating in vertical orientation, the total holding force would need to be made substantially greater than the weight of the armature. Even in horizontal orientation, simple inertia and static friction alone are insufficient to prevent shifting away from the stroke-end position due to vibration, acceleration, etc. In a vehicular lock, for example, if the bolt were not held at the stroke-end positions, it could tend to shift away from the end position due to vehicle movements, acceleration and/or vibration.
Especially in fields such as aerospace, missiles and other defense products, the actuator must be able to deliver a high transfer load and to remain stable in the end conditions under adverse ambient conditions including high acceleration.
Furthermore, it is generally preferable for the actuator to have only two terminals, especially in situations such as remote control that require long wiring runs. A bistable device can be made to operate from two wires if it is made to reverse its stroke by reversing the polarity of the DC.
Such requirement are not satisfied by a simple electromagnet and armature such as in a simple relay or solenoid, since the armature motion cannot be reversed by reversing the current; furthermore, even if the armature is made stable at one stroke-end when the coil is unpowered, the other stroke-end would require continuous electric power or some other special provision such as a spring-loaded mechanical toggle to hold it in the xe2x80x9conxe2x80x9d condition.
Commercial electromagnetic linear actuators are made in an economical and efficient solenoid form having a coil that is able to move an iron core or armature, typically in the manner of a plunger. Basically, whether AC or DC powered, ordinary solenoids can only draw the armature back into its central xe2x80x9chomexe2x80x9d position relative to the coil from either of two opposite offset initial locations; the armature must be returned to either of the initial offset locations by some other force such a gravity or metal springs. Also there are large swings of coil impedance due to large variations in the magnetized air gap separation and in flux density, which make the electrical driving system inefficient and difficult to design.
As an alternative to costly rotary electric motors and/or other costly complex mechanisms, it is known in the art to obtain bi-stable operation utilizing two complete electromagnetic actuators working in opposite directions and controlled by selecting between one coil for A-B and the other coil for B-A actuation: this actuator system would require more than two electrical terminals and connecting wires. Such a system could be further evolved to utilize a common armature or core that can be driven in either direction by selecting one of two coils.
Positive holding force at both stroke-end positions could be implemented by deploying a pair of permanent magnets, located to each act at respective opposite ends of the stroke.
Operating on DC and introducing at least one permanent magnet into the main motive function of the actuator enables travel reversal to be accomplished by current direction, and opens up the possibility of also utilizing the permanent magnetism to provide the necessary end holding force to hold the armature in one or both of its two stable end positions. However, in known art, such a system is expected to require two permanent magnets: one for each of the two stable stroke-end positions.
Stroke-end holding force in conventional vehicular locks is often implemented by some form of mechanical force such as from metallic springs in coiled or other form.
Patents showing mono-stable lock actuators utilizing a single solenoid with spring bias are exemplified by U.S. Pat. Nos. 3,576,119, 4,917,419, 4,907429 and 4,679,834.
U.S. Pat. Nos. 5,199,288 and 4,703,637 exemplify actuators that obtain bistable stroke-end positions for locking and unlocking purposes through the use of a rotary electric motor typically utilizing a worm gear engaging a threaded shaft or pinion.
U.S. Pat. No. 5,231,336, by the present inventor, discloses a mono-stable electromagnetic actuator for active vibration control. The magnetic armature of this actuator operates in the voice coil mode to create a linear vibratory motion under the influence of a sinusoidal current through the surrounding coils. A positive current in the coils drives the armature in one direction while a negative current drives the armature in the opposite direction. Removing the current returns the armature to its stable central rest position under influence of the magnetic field and internal springs. This construction is inherently mono-stable at the center position: it would require radical redesign to provide a stable unpowered armature position on each end of the stroke.
U.S. Pat. No. 4,829,947 by Lequesne for variable lift operation of a bistable electro-mechanical poppet valve actuator discloses an automotive valve actuating device whereby a valve, with attached armature is spring-biased toward a neutral central position but held in a full open or a closed position by permanent magnets having associated coils. Activation of a coil can fully cancel the field of the associated magnet to allow the spring to move the valve to the other position.
U.S. Pat. No. 4,533,890 to Patel discloses a PERMANENT MAGNET BISTABLE ACTUATOR for automotive valves, having a pair of solenoid coils acting on a common central core which requires two coaxial permanent magnets to provide bi-stability.
It is an object of the present invention to provide an actuator, suitable for locking purposes in missiles, aerospace craft and the like, providing bistable operation with two unpowered opposite stroke-end conditions that are positively stabilized by design-controllable end-holding force to withstand designated axial acceleration loading.
It is a further object of the present invention to make the actuator simple and inexpensive and in a basic coaxial form that utilizes a minimum quantity of coils and permanent magnets, preferably only one of each.
The aforementioned objectives have been accomplished in the present invention of a bistable actuator in a coaxial plunger-type configuration having a single coil in a shell/yoke surrounding a single armature containing a permanent magnet. The actuator performs its transducing function primarily in the manner of a loudspeaker voice coil, i.e. it is driven to move through a linear stroke by the force from magnetic action on those turns of the current-carrying coil that are at that instant located in the substantially constant magnetic flux path through a radial magnetic gap traversing the coil. In the case of the loudspeaker, the voice coil and cone assembly are suspended as a movable mass portion for purposes of the required vibration, while the PM (permanent magnet) system is made to be the stator, i.e. the fixed mass portion. However, in the present actuator the foregoing loudspeaker structural arrangement is reversed: the magnet is made to be the main part of the movable mass portion, i.e. the armature, and the coil assembly is made to be part of the fixed mass portion, i.e. the shell/yoke/stator, thus avoiding the need for flexible electrical connections that are required in a loudspeaker for connecting the voice coil.
A cylindrical shell serves as a magnetic yoke that cooperates with the armature magnet to provide bistable stroke-end locations of the armature, and that cooperates with the coil and magnet in a manner to motivate actuation between these two stroke-end locations when the coil is powered.
The shell and the bobbin are configured in a special manner that locates the coil in essentially one end half-portion of the shell while a tubular channel formed integrally with the bobbin extends full length of the shell. The tubular channel is dimensioned internally to provide a sliding fit with a pair of circular pole plates one on each end of the magnet, thus guiding the armature-plunger in an axial travel path within a designated stroke length. The armature can be shifted to the opposite end of the stroke by energizing the coil with DC (direct current): the direction of armature movement depends on the DC polarity, so that only two terminals and two connecting wires are required.
For all armature locations within the main central portion of the stroke range, the rim of one of the circular pole plates on the magnet forms a primary working magnetic air gap of substantially constant pole-face separation from the inside surface of the shell, with the radial PM flux from the magnet acting on the turns of the coil in that region. The PM flux path returns through a secondary magnetic air gap between the rim of the other circular pole plate at the opposite end of the magnet, and a region of the shell that is stepped down to a substantially smaller inside diameter in that end portion so as to maintain a constant separation and PM flux density at the secondary return magnetic gap, so that the actuator functions primarily in a voice-coil mode over the main central portion, i.e. about 90%, of the stroke, and transitions to a magnet-keeper-attraction mode at the stroke-end regions for bi-stability.
The stable PM attraction forces in the two stroke-end positions can be controlled in design by area of contact and thickness of the soft iron pole pieces, the shell-to-pole plate spacing, and/or the optional introduction of a controlled-thickness spacer of non-magnetic material at either end.